1. Field of the Invention
The invention is related to the field of decorative ceramic tiles and in particular to ceramic tiles having a heating element formed on the side opposite the side that is visible when the ceramic tile is cemented to a floor, wall or ceiling.
2. Description of the Prior Art
EU-A-0 158 091 discloses a space heating element composed of a ceramic shaped body provided on the side facing away from its visible side with a heating conductor extending across its surface in the form of an electrically resistive coating consisting of an electrically conductive ceramic glaze. The electrically resistive coating is a resistive layer made of a material in which nonmetallic electrically conductive particles which have a large specific area and do not substantially alter their electrical conductivity at higher temperatures are embedded in an electrically nonconductive or poorly conductive carrier substance. The electrically resistive coating is applied in such a way that the resistive coating has even electrical and thermal conductivity. A special form of such a space heating element is, for example, a large-area wall, floor or ceiling tile.
Under unfavorable conditions, tiles having an electrically resistive coating consisting of an electrically conductive ceramic resistance glaze may suddenly come off. This makes it problematic to attach such tiles to ceilings or even walls by means of adhesive because additional means of attachment such as clamps or hooks would be required, however this is often impossible or at least undesirable.
According to EU-A-0 158 091, the electrically resistive coating is contacted in particular by contacting elements disposed symmetrically on the electrically resistive layer. However, it has turned out that such elements provided on the heating layer tend to come off and also fall to ensure a sufficiently even passage of electricity across a surface.
The problem of insufficient adhesion also occurs, surprisingly enough, between the conductive resistive layer and an insulating layer by means of which the lining element is attached to the attachment base.
There is up to now apparently no insulating material, in particular organic insulating material, which enters into a permanent and reliable bond with the conductive resistive layer.
The invention is based on the problem of designing the electrically conductive resistive coating of the lining element in such a way as to prevent it from coming off either the attachment base directly or an insulating layer thereabove, and also so as to bring about an improvement in the heating properties of the space heating element.
The problems discussed above are solved according to the invention by covering the resistive layer present in the form of an electrically conductive resistance glaze by a layer of nonconductive or poorly conductive ceramic glaze with a carrier substance having the same composition as the resistance glaze but with incorporated kaolin particles, and heating the formation consisting of the ceramic shaped body, the electrically conductive ceramic glaze and the ceramic cover layer to a temperature below the quartz transition point (&lt;750.degree. C.), thereby fusing the resistive layer and the cover layer into the ceramic lining element.
Surprisingly, every organic insulating coating adheres without any problem to a lining element coating in this way.
Another surprising effect of the proposed solution is that the electrical resistance of the covered resistive layer, i.e. the heating layer, is considerably reduced. This is due to the fact that if there is a suitably selected surplus of kaolin particles relative to the glass parts of the carrier substance, glass parts come out of the resistance glaze (heating layer) during firing of the overall formation and are bound into the cover layer. This reduces the insulating glass substance in the resistance glaze which has an adverse effect on the electrical conduction of the resistance glaze, thereby reducing the electrical resistance of the resistance glaze, i.e. the heating layer. One thus obtains an improvement in the heating properties of the space heating element due to the embedding of kaolin particles in the cover layer.
For example, with 30% by weight of kaolin one observed a resistance reduction to 50% compared to that of the same formation without the addition of a cover layer.
This phenomenon has been utilized according to the invention in such a way that the amount of kaolin particles of the cover layer is selected so as to obtain a certain change of conductivity which in turn allows for a distinct reduction in the amount of electrically conductive particles in the heating layer, without altering the originally desired electrical resistance of the heating layer, i.e. without a cover.
With respect to the electrical contacting elements provided on the heating layer, an even passage of electricity across the surface is achieved by contacting the electrical heating layer in such a way that the contact elements or contact layer is applied directly to the ceramic shaped body, this layer growing in ramp-like fashion toward the outer edge of the ceramic shaped body with the ramp height dimensioned in accordance with the thickness of the heating layer and the ramp width more than fifty times this thickness. In one embodiment the heating layer thickness was 120.mu. and the width of the contact layer 15 mm. This measure also reduces the tendency of the contact elements or contact layer to come off.
A further advantage of this proposed solution is based on the application of the contact layer directly to the back of the ceramic shaped body as the first procedural step, which is followed by the application of the electrical heating layer as the second step. The third step is to apply the cover layer and in a fourth step the entire formation is subjected to a firing or heating process.